Incomplete oracle failed assertion error messages

[nventuro]

Problem

When developing tests and writing oracle mocks it is sometimes hard to figure out why execution fails. An example of this is the mock returning an incorrectly sized array, in which case we only get

error: Assertion failed: 'Foreign call result array doesn't match expected size'

The error message is good in that it points at the right callsite and oracle, but it'd be great if it could also display more information such as the expected size, actual size, or even actual contents of the oracle response.